Use of "HAVE TO", "MUST" and "SHOULD"

DIFFERENCE BETWEEN "HAVE TO"  AND "SHOULD"

Look at the following example

• "Nurses have to wear a uniform" 
• "I am not feeling wel but I have to go to schol because I have a test.

( "have to" shows compulsion).

but when I say "I should attend school every day." or "I should save money for rainy days."

(Should shows desirable condition of something which you understand your moral duty)

Both "have to" and "must" mean obligation 

• "Must" refers to obligation from 'inside'(depends on person speaking or you understand it is necessary)
• "have to" refers obligation from 'outside' (i.e. a regulation or order from outside or you are forced to do so.)

Example-: You have to bring your I-cards to the examination hall.

SHOULD as model verb showing suggestion.

• SHOULD + Verb (I)  used to suggest advise someone for something.

LOOK AT FOLLOWING LINES CAREFULLY.

• I am sure I should go home today.
• It is raining you should take an umbrella or wear rain suit.

Give three suggestion to your friend for each situation

• He has fever.
• He has caught cold.

Make a list of New Year resolution using "Have to" "Must" and "Should"

• To give up bad company
• To get job